Ever since marijuana became legal in Colorado and Washington on January 1, the idea of legal pot smoking nationwide went from something President Obama made fun of to a serious national controversy. Most of the conversation has been about legal weed’s effect inside America, be it in terms of individual rights, public health, or the horrific and frankly racist cast of the current anti-weed punishment regime.

But the effects of America’s drug war aren’t confined to our borders. Drug prohibition in the world’s largest drug market plays a critical role in shaping the business of global terrorism. Here’s what we know about the critically important, but underdiscussed, ways that the war on drugs is complicating the war on terrorism.

Narcoterrorism, the use of profits from the illegal drug trade to finance terrorist activities, is common around the world. Sometimes, terrorist groups turn to drugs for extra revenue — the FARC in Colombia and the Taliban are the classic cases here. In other cases, drug cartels turn to terrorism to secure their revenue stream — think Mexican groups like the Sinaloa cartel. Either way, though, there’s a clear and increasingly tight relationship between illicit drug profits and terrorism.

A quantifiable one, in fact: more drug production seems to predict more terrorist attacks. James Piazza, a political scientist at Penn State, studied the relationship between the supply of coca and poppy and terrorist attacks. After controlling for confounding variables, Piazza found that, in 170 countries around the world, the things that made the drug trade more profitable — like higher volumes of coca/poppy production — were significantly correlated with higher levels of terrorist violence one year later. When you give violent organizations more money, as it turns out, they tend to spend it on violence.

You might be surprised that there’s a connection between terrorist attacks and drugs for a 170 country sample, given that coca and heroin are overwhelmingly grown in Colombia and Afghanistan. But that’s taking a narrow view of the drug trade, which is global in scope. Since there’s demand for drugs everywhere, and it’s easier for local groups to sell product grown elsewhere than for foreigners to move in, terrorists around the world have an incentive to get in the drug game in their corner of the world.

Hezbollah, for instance, pockets millions by distributing drugs from South America in Africa and the Middle East. The Irish Republican Army and some of its splinter groups have profited from the narcotics trade, and the Turkish Kurdistan Worker’s Party (PKK) has played a huge role in the European narcotics trade. Because illegal drugs flourish in the same places and spaces that terrorist organizations do — the poorly governed, poorly policed global shadows — a mutually beneficial relationship between drug dealers and terrorists emerges with alarming frequency.

Now, Piazza’s study also correlated aggressive attempts to eradicate the drug supply to reduced terrorist activity, but that’s a bit confusing. The reason is that in Colombia and Afghanistan, efforts to destroy drug fields go hand-in-hand with broader counterinsurgency efforts. When the Colombian and Afghan governments (in cahoots with their mutual ally, the United States) launch drug eradication campaigns, they’ve historically taken place at the exact same time as broader military efforts against the FARC and the Taliban, respectively. So really, all this second finding of Piazza’s shows is that attacking insurgents can reduce insurgent violence, at least in the short term.

As we all know, though, short-term gains don’t always translate into long-term victories — and nowhere is that more true than the war on drugs. “By 2009, eradication had the following effects: It did not bankrupt the Taliban,” according to Vanda Felbab-Brown, an Brookings drug trade scholar. “Rather, eradication strengthened the Taliban physically by driving economic refugees into its hands,” as there was no equally lucrative alternative to poppy cultivation for Afghan farmers looking to feed their families. In 2009, the Obama Administration conceded eradication’s failure, ending centrally planned efforts to destroy poppy fields. Today, no one seriously thinks force can stop the Afghan opium trade.

In fact, the most significant “accomplishment” of America’s decades-long war on Colombian coca was the birth of the ultra-violent Mexican cartels we live with today. That the offensive in Colombia midwifed the modern cartel isn’t a fringe opinion, but rather a truth so plain as to be unremarkably asserted in the middle of a Congressional Research Service report on the Mexican cartels. “As Colombian [cartels] were forcibly broken up, the highly profitable traffic in cocaine to the United States was gradually taken over by Mexican traffickers,” June Beittel, a CRS Latin America analyst, wrote. “As Mexico’s drug trafficking organizations rose to dominate the U.S. drug markets in the 1990s, the business became even more lucrative. This ‘raised the stakes,’ which encouraged the use of violence in Mexico to protect and promote market share.”

As a consequence, roughly 60,000 Mexicans have been murdered in organized crime related homicides since 2006. By any understanding of the term, the cartels — who were never peaceful — have become major terrorist organizations, employing car bombs and assassinations to terrorize the Mexican population and suborn the government to cartels’ whims. This isn’t just a Mexican concern: even if you think the United States bears no moral responsibility for a situation it created, a failed narco-state on America’s southern border is definitely a national security priority.

So if illict drug profits are helping terrorist groups slaughter innocent people, and counter-narcotics enforcement has eminently failed to solve the problem, what can we do? Well, the logic linking drugs and terrorism depends crucially on drugs being illegal. If the drug trade didn’t take place in the shadows, there would be no reason for farming to take place in failed states or for drug sellers to partner with terrorists for distribution. “Opium poppy, marijuana, and even coca grow in a broad range of countries, not just those where production currently occurs,” Harvard economist Jeffrey Miron writes. “If drugs were legal, production would be widely dispersed and have no particular overlap with countries that harbor terrorists.”

Miron’s simple argument is impossible to assess in the aggregate; each drug market has different connections to terrorism, so the effect might be different depending on the substance. Instead, let’s instead pare down and focus solely on marijuana, the only drug whose legalization is possible in the United States in the near term. A 2010 RAND Corporation study found that if legal American marijuana replaced Mexican imports, either through national legalization or a national grey market birthed by state-level legalization, cartels would lose a full 20 percent of their drug income. A recent revision of the RAND study by a Mexican think tank came to a slightly higher estimate, adding that “losing marijuana revenues could have a transformative impact on the Mexican drug trafficking industry, over and beyond the direct potential reduction of marijuana export income.”

Afghanistan, not often discussed in marijuana legalization debates, might also see real gains. A 2009 U.N. Office of Drug Control (UNODC) report found that Afghanistan produced a huge percentage of the world’s cannabis and the largest percentage of hashish (cannabis resin). The Afghan marijuana trade, valued at $65 million in 2012, is “taxed by those who control the territory, providing an additional source of revenue for insurgents,” according to UNODC Executive Director Antonio Maria Costa.

There are some questions as to whether reducing drug revenue would actually reduce terrorist violence. The RAND researchers, for instance, suggest Mexican cartel violence might actually increase in the short term as gangs struggled over scarcer resources. However, the best historical analogy RAND could uncover — the American mafia post-prohibition — suggests the long-term reduction in violence could be enormous. Like cartels, the mafia engaged in all sorts of profitable illegal enterprises beyond the illegal intoxicant racket, the loss of alcohol revenue was seemingly devastating for the mafia. Homicides declined rapidly after the repeal of prohibition; “plausibly,” RAND’s researchers write, “a large share of that decline was accounted for by fewer killings in the bootlegging trade.”

Piazza’s findings, published after RAND’s paper, are also crucial here. His research suggests a direct correlation between drug profits and terrorist violence; increases in the latter directly follow increases in the former (a point he solidified in a follow-up study on opium in Afghanistan). That suggests each dollar in drug profit at least marginally increases the ability of cartels and other terrorist organizations to do their nasty violence; soldiers and car bombs don’t pay for themselves. Even though cartels and the Taliban don’t need marijuana profits to be violent, they’ll be somewhat more limited in their ability to conduct violence without them.

Whether or not you support legalization, one thing is clear: our current drug policy is creating real opportunities for terrorist groups to fund their violent activities. Any serious approach to the drug/terrorism connection needs to begin from the starting point that prohibition, as it implemented right now, isn’t working.

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